High performance mixed signal A/D circuits require improved power distribution and decoupling as compared with conventional circuit devices. In addition, the inductive and capacitive parasitic losses present in conventional single chip packaging and surface mount technology dictate that many new high performance circuit designs be implemented using multi-chip module (MCM) packaging technologies.
A multi-chip module (MCM) is a single electronic package containing more than one IC. An MCM combines high performance ICs with a custom-designed common substrate structure which provides mechanical support for the chips and multiple layers of conductors to interconnect them. This arrangement takes better advantage of the performance of the ICs than does interconnecting individually packaged ICs because the interconnect length is much shorter. The defining characteristic of MCMs is the complex substrate structure that contains the circuit pattern that interconnects the ICs and which is fabricated using multi-layer ceramics, polymers, silicon, metals, glass ceramics, laminates, or other suitable materials.
Multi-Chip Modules (MCMs) offer a way to pack more integrated circuits into a given module surface area. In addition to reducing size, this technology permits increased speed because it shrinks interconnect distances. A typical MCM has bare ICs mounted on a high-density interconnect (HDI) substrate encapsulated within an environmentally-protected package.
Types of MCMs include: MCM-L (laminated PC board), MCM-C (co-fired ceramic), MCM-D (deposited thin film), and MCM-Si (silicon substrate). MCM-C technology is based on ceramic dielectrics, noble metals and thick film processing. MCM-L technology is based on organic dielectrics, plated copper metallization and laminate preceding. MCM-D technology is based on deposited dielectrics, copper or aluminum metallization and thin film processing. More formal definitions for these primary types of MCMs have been established by the Institute for Interconnecting and Packaging Electronic Circuits (IPC). In particular, MCM-L are understood as modules which are constructed of plastic laminate-based dielectrics and copper conductors utilizing advanced forms of printed wiring board (PWB) technologies to form the interconnects and vias. They are commonly called “laminate MCMs”. MCM-C modules are understood as modules which are constructed on confided ceramic or glass-ceramic substrates using thick film (screen printing) technologies to form the conductor patterns using fireable metals. The term “cofired” implies that the conductors and ceramic are heated at the same time. These are also called thick film MCMs. MCM-D modules are understood as modules which are formed by the deposition of thin film metals and dielectrics, which may be polymers or inorganic dielectrics. These are commonly called thin film MCMs.
From the above definitions, it can be understood that MCM-Cs are descended from classical hybrid technology, and MCM-Ls are essentially sophisticated printed circuit boards. On the other hand, MCM-Ds are the result of manufacturing technologies that draw heavily from the semiconductor industry.
The application of MCM technologies for developing high performance, mixed signal circuits aids in addressing the interconnection challenges that arise when developing these devices. The high switching speeds, high bandwidth, and high dynamic range of these circuits require that the power/ground distribution systems provide very low impedance decoupling with very low noise and ripple. The AC and DC loss characteristics of the substrate signal interconnect structure must be low and must provide sufficient signal, power, and ground layers to accommodate both analog and digital power and ground planes. In high speed circuits where substantial current switching is occurring, the decoupling performance is directly affected by the series inductance between the capacitor elements and the power and ground planes. Voltage spikes caused by L*di/dt effects will result in voltage differences in the power and ground planes. Low inductance surface mounted capacitors have been developed, however, electrical simulation results of standard MCM structures with surface mounted low inductance chip capacitors indicate that these structures still behave as LC transmission lines allowing the propagation of waves across the ground plane. Voltage differences of greater than 30 mV were obtained in simulations.
Present commercially available MCM interconnect solutions are comprised of the three basic MCM technologies, MCM-C, MCM-L, and MCM-D. Generally, these technologies rely on surface mounted capacitor chip components to provide decoupling capacitance for the IC devices. Although, this technique can work well, the current switching demands of high speed circuits can still pull sufficient current through the power lines to cause voltage spiking and ground bounce.
A better solution is to distribute the decoupling capacitance by placing the power and ground layers next to each other separated by a thin dielectric layer. This makes the decoupling capacitance integral to the substrate structure and provides the lowest series inductance.
However, in most cases this integral distributed decoupling capacitance is too small to be sufficiently effective due to the relatively low dielectric constants of the dielectric materials being used and the relatively large spacing between the power and ground layers. One specific MCM-D technology, manufactured by nChip Inc. utilizes multi-layer thin film processing with aluminum metallization and SiO2 dielectric fabricated on a silicon substrate. This technology is described in U.S. Pat. No. 5,134,539. However, this MCM-D technology is typically limited to 4 metal layers consisting of 1 power plane, 1 ground plane, and 2 signal layers, and therefore, fails to provide a sufficient number of layers to create an interconnect layer that allows separation of the analog and digital sections for mixed signal application. Additionally, the aluminum metallization applied by this process is more resistive than equivalent copper metallization, and therefore results in RC losses in the signal traces.
Therefore, a need remains in the art as none of the existing interconnect structures provide the features required or desired for high performance mixed signal A/D circuits.